As a material having thermal insulation properties with a low thermal conductivity coefficient, a silica aerogel has been known. A silica aerogel is useful as a performance material having superior performance (thermal insulation properties, etc.), unique optical properties, unique electrical properties, etc., and has been used, for example, as an electronic substrate material utilizing an ultra-low dielectric constant characteristic of a silica aerogel, a thermal insulation material utilizing high thermal insulation properties of a silica aerogel, a light reflection material utilizing an ultra-low refractive index of a silica aerogel, etc.
As a method for producing such a silica aerogel, a supercritical drying method has been known, by which a gel compound (alcogel) obtained by hydrolysis of an alkoxysilane and followed by polymerization is dried under a supercritical condition of a dispersing medium (for example, refer to Patent Literature 1). A supercritical drying method is a method for removing a solvent contained in an alcogel by placing an alcogel and a dispersing medium (a solvent for drying) in a high pressure container, and transferring the dispersing medium into a supercritical fluid by applying a temperature and a pressure at or above a critical point thereof. Since, however, a supercritical drying method requires a high pressure process, a large capital investment in special equipment resistant to supercritical conditions, or the like is necessary, and further a lot of time and efforts are necessary.
Therefore, a technique for drying an alcogel using a general purpose method not requiring a high pressure process has been proposed. As such a method, for example, a method, by which a monoalkyltrialkoxysilane and a tetraalkoxysilane are used in a combination at a specific ratio as a gel source material such that an alcogel obtained may have an improved strength, and may be dried at a normal pressure (for example, refer to Patent Literature 2). However, when such normal pressure drying is used, a gel tends to contract due to a stress caused by capillary force in the alcogel.